Remotely actuated polymer nanocomposites--stress-recovery of carbon-nanotube-filled thermoplastic elastomers.
Stimuli-responsive (active) materials undergo large-scale shape or property changes in response to an external stimulus such as stress, temperature, light or pH. Technological uses range from durable, shape-recovery eye-glass frames, to temperature-sensitive switches, to the generation of stress to induce mechanical motion. Here, we demonstrate that the uniform dispersion of 1-5 vol.% of carbon nanotubes in a thermoplastic elastomer yields nanocomposites that can store and subsequently release, through remote means, up to 50% more recovery stress than the pristine resin. The anisotropic nanotubes increase the rubbery modulus by a factor of 2 to 5 (for 1-5 vol.%) and improve shape fixity by enhancing strain-induced crystallization. Non-radiative decay of infrared photons absorbed by the nanotubes raises the internal temperature, melting strain-induced polymer crystallites (which act as physical crosslinks that secure the deformed shape) and remotely trigger the release of the stored strain energy. Comparable effects occur for electrically induced actuation associated with Joule heating of the matrix when a current is passed through the conductive percolative network of the nanotubes within the resin. This unique combination of properties, directly arising from the nanocomposite morphology, demonstrates new opportunities for the design and fabrication of stimuli-responsive polymers, which are otherwise not available in one material system.